Multi-User Multiple Input Multiple Output (MU-MIMO) technology has been recognized as a technique to boost spectrum efficiency without significantly increasing infrastructure costs. For LTE-A Rel. 10, transmission mode 9 (TM9) has been introduced to support up to an 8-by-8 MIMO transmission. That is, eight Demodulation Reference Signal (DMRS) antenna ports are defined to demodulate up to rank 8 transmissions. Nevertheless, such an approach does not provide higher-order MU-MIMO modes.
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